Barack Obama and his Democratic allies have scored a series of victories in primary races while the Republican establishment has lost out to agitated Tea Party supporters, bolstering the president's hand in November's mid-term elections.

The results of primaries in four states on Tuesday will leave moderate Democrats facing Republicans who have been tainted by extreme views, or accusations of unethical conduct, in key races for the US Senate and state governorships. This should undermine the Republicans' attempts to retake control of Congress. "This is the best night the Democrats have had this year," said Larry Sabato, a political science professor at the University of Virginia. "They've improved their position in all four states and the Republicans weakened theirs. The Democrats couldn't have written a better script."

Crucially for Obama, Senator Michael Bennet won a clear victory for the Democratic nomination against Andrew Romanoff, who had Bill Clinton's backing and was billed as likely to draw strong support from discontented liberals. Obama had campaigned heavily for Bennet, who won by with a nine-point lead, so the result undermines claims that the president is a liability for candidates.

Bennet's prospects have been helped by the decision of Colorado Republican primary voters to elect Tea Party supporter Ken Buck as their candidate for Senate. Buck, who beat the Republican leadership's favoured candidate, is thought likely to alienate many independent voters.

The Republicans elected a candidate for governor of Colorado, Dan Maes, who is widely viewed as a liability to the party because of his extreme conservative views, including a claim that a bicycle sharing scheme in Denver is part of a United Nations plot to take over the city. Republican leaders backed an establishment candidate who was thought to have a better chance against the Democrats.

The Colorado races appear to show the anti-Washington sentiment said to have swept the country is working more against Republican insiders than Democrats.

"Colorado is the big one," said Sabato. "It has strengthened Obama's position. The Republicans turned down Jane Norton, who could have won the Senate race in November, for a Tea Party candidate, Ken Buck. The Republicans have also given it away on the governor."

The results of primaries in three other states – Connecticut, Georgia and Minnesota – also appeared to favour the Democrats in November.

In Connecticut, Linda McMahon, the former head of World Wrestling Entertainment, won the Republican nomination for the US senate to run against Democratic challenger Richard Blumenthal, who is accused of falsely claiming he served in Vietnam.

McMahon won after spending $20m of her own money.

"Connecticut Republicans today nominated a corporate CEO of WWE who, under her watch, violence was peddled to kids, steroid abuse was rampant, yet she made millions," said Robert Menendez, chairman of the Democrats' Senate campaign committee.

In Georgia, the Republican vote in the race for governor split after a vicious campaign between Nathan Deal, a former congressman who resigned amid allegations of corruption, and Karen Handel, who was portrayed as not conservative enough on issues such as abortion and same-sex marriage.

The Republican leadership favoured Handel but Deal won by a wafer-thin margin by portraying himself as anti-establishment despite being named by one watchdog as among the 15 most corrupt members of Congress. He resigned as a member of the House of Representatives this year after an ethics investigation concluded that he had improperly used his office to direct hundreds of thousands of dollars worth of state funds to his family's car salvaging business.

However, Sabato noted that despite the restless mood among some voters, and the influence of the Tea Party, establishment candidates will dominate November's races.

"The Tea Party candidates have undermined the Republicans in some states and districts but not in most," he said. "The renomination rate is 98%. Some of the Tea Party candidates will win but would it have been better for the Republicans to have mainstream candidates? Absolutely. No question about it."